The present disclosure relates to exhaust stream treatment, and more particularly to systems and methods for the reduction of nitrogen oxide emissions.
Exhaust gases from internal combustion engines are a source of air pollution when discharged into the atmosphere. One class of pollutants, collectively referred to as nitrogen oxides (NOx), includes nitric oxide/nitrogen monoxide (NO) and nitrogen dioxide (NO2). These gases are known to be precursors to smog. Accordingly, progressively more stringent regulations are being implemented in an effort to reduce, and eventually eliminate, the concentration of NOx in exhaust emissions.
Numerous methods to control NOx emissions have been proposed. One particular area of focus has been on treatment of the post-combustion exhaust stream to produce less regulated gases that subsequently may be released into the atmosphere. Selective catalytic reduction (SCR) systems require the addition of a chemical reagent, which is a reducing agent or reductant, to mix and react with the exhaust stream over a catalyst to convert the NOx into unregulated gases such as nitrogen, oxygen, carbon dioxide if it is carbon based reductant and/or water vapor.
In an ideal system, the NOx and reductant concentrations would be exactly equal to their stoichiometric values, uniformly across the entire exhaust stream with a sufficiently long catalyst residence time (i.e., exposure time of reactants to catalyst), effective for complete reaction of the NOx and reductant to the unregulated gases. The result would be zero NOx and zero reductant leaving the catalyst and released into the atmosphere. However, in practice, there are several limitations in achieving uniform concentrations. One such limitation is the spontaneous partial oxidation or combustion of the reductant, in the presence of oxygen, at point sources immediately after introduction into the hot exhaust stream. This can result in inefficient usage of the reductant and inefficient reduction of NOx.
Accordingly, many challenges exist for NOx-reducing emission control technologies. One such challenge is to develop a method that maximizes the reduction of NOx to unregulated gases, while ensuring safety and minimizing inefficient use of the reductant.